If you are reading this, you are probably thinking of or already replacing an FAS in your formulation in anticipation of the pending regulatory moves being telegraphed by EPA and ECHA.  You probably know that this will not be an easy task.  FAS have been used for decades and have several unique performance properties that have allowed them to fill niches where nothing else quite does the job.

Step one is to take some deep breaths, do some hot yoga, eat a gummy bear, or otherwise perform your personal calming ritual.  It is going to be okay – mostly. The parallels in surface energy, incompatibility, and interfacial tension between FAS and PDMS (silicone) based products often make them the “next best thing”.  In many cases, silicon-based solutions will allow you to achieve your goal.

Let’s examine the unique fluoro-properties one at a time starting with low surface tension.  The surface energy of various fluoroalkyl compounds are in the range of 13-20 mN/mi. FAS surfactants reduce the surface tension of aqueous solutions to a similar range with very low critical micelle concentrations (CMC).  That low CMC results in use levels that are 1-2 orders of magnitude lower than alternatives but the higher costs of FAS offset the low use level on an economic basis. 

In many of these applications, organofunctional silicones are an excellent option for replacing an FAS.  The silicone polymer used as a backbone in the chemical hybrids of silicone and organic moieties has a surface energy of 20 mN/m.  This is only “beaten” by FAS compounds at 13-20 mN/m.  As benchmarks, Teflon is 19 mN/m and hydrocarbon-based materials are 30-40 mN/m.  There are no other substances with surface energies between those of PDMS and FAS materials. 

Organosilicone materials derived from the PDMS backbone have surface tensions of 20-35 mN/m. One can chemically append hydrophiles creating surfactants which reduce the surface tension of aqueous systems as low as 20.5 mN/m. These can be designed to either control or stabilize foam as well.

In Siltech’s labs, we have experience evaluating our silicone-based materials in many of these applications.  AFFF, wetting, release applications, stain protectants, textile treatments, antifoams, car care, HI&I, coatings, dispersing, glass treatments, leather treatments, dental, pulp and paper processing, cosmetics and hydrophobic treatments have all been evaluated with favorable outcomes. 

Another unique FAS property is low surface energy which provides water, oil and stain repellencyii.  We have recently published a review of different Siltech methods for generating water repellency. The basic film forming silicone emulsions give an effective, if not exciting, 85° contact angle on glass.  This improves as the emulsifiers are rinsed out of the network. 

More effective aqueous methods have been demonstrated.  The highly cross linked Silmer® Q resins can be delivered from an evaporating solvent; in a sol-gel formulation; or in Siltech E-2199 emulsion.  These approaches give 115° contact angles on glass. 

In another approach, we can achieve this level of water repellency with silicone quaternary ammonium compounds.  These can offer the added benefit of quick exhaustion onto glass, textiles, or other surfaces from solution. Finally, Silmer TMS products can also obtain 115° on glass.

Perhaps the most unique thing that FAS do is repel oils.  Oleophobicity is considerably more challenging than hydrophobicity.  The FAS are perhaps so effective because they are so immiscible with oils which increases the repellency.  Silicones are also immiscible with oils but simple organosilicone treatments are usually relatively ineffective at providing this oleophobicity.

We have found that Silmer OHT linear organosiliconeiii materials offer good oleophobicity as measured by stain resistance.  While the results are not quite at the level of FAS compounds there is significant performance improvement, comparable to that of our Fluorosil® perfluoro C4F8 based fluoroalkyl silicone products.  We see similar results with our species which also have chemical anchors on the organic appendages

We have also found that some high Tg modifications, either hydrocarbon wax or EO chains, provide oleophobicity at room temperature.  We treated cardboard sections with a series of Silsurf® silicone polyether derivatives or Silwax® hydrocarbon modified silicones.  A few drops of vegetable oil were placed on the treated cardboard samples and evaluated over an hour at ambient conditions. 

The control and many of the treated cardboard sections absorbed the vegetable oil over the hour.  However, cardboard treated with Silwax D232, which has a waxy hydrocarbon chain and is a high melting wax itself, gave very good beading of the vegetable oil after one hour. Similarly, the Silsurf A017-UP, which has a waxy EO chain and is a low melting wax (~40°C) also shows strong beading with no absorption after the hour. 

Recent technical service work has shown interesting results with some organosilicones at repelling sunflower oil from a proprietary tile grout. Siltech E-4135, an amino-functional film forming emulsion; Siltech E-2155, a silicone film forming emulsion; and Silmer TMS emulsion all gave good repellency. Cross-linking is a consistent theme in these success stories. 

In fact, the cookware industry has replaced Teflon coated cookware with ceramic coated pots and pans for effective oleophobicity, heat resistance and release.  Ceramics are silicon-based matrices highly cross-linked in very high heat. 

Following this idea, we have done some work with our Q resin materials.  These also cross-link in four directions offering cross-link densities in the direction of those found in high heat cured ceramics. While this is a nascent area of research, we are intrigued by the early results.  Silmer DTQ-75 increased the contact angle of sunflower oil on aluminum from 50° to 55° and showed excellent beading of sunflower oil in the cardboard test described above.

Due to the extremely strong bond strength of the C-F linkage, FASs are very stable to heat and chemical degradation.  While silicone itself is pretty good at heat resistance, the Si-O-Si bonds are very labile to chemical degradation in an acid/base sense.  One could look at it that the strong C-F stability is why FAS tend to build up in the environment and the hydrolytic lability of PDMS is its main mechanism of degradation in the environmentv. 

The PDMS base polymer is thermally stable to about 300°F.  However, adding organic groups to the silicone to gain solubility, reactivity, and other properties reduces the thermal stability to that of the organic moiety.  In other words, the PDMS does nothing to stabilize the heterolysis of C-H bonds in the organic appendages. 

Regarding chemical resistance, Leatherman et.al.vi  published carbosilane analogues to PDMS surfactants.  This is elegant work which I will simplistically describe as replacing the O atoms of PDMS with CH2 groups.  Because the hydrolytic lability of siloxane polymers is due to breaking the Si-O-Si bonds, this invention creates an acid/base stable polymer with very similar surface energy properties to PDSM derivatives. 

At Siltech, we are again looking to Q resins to improve this property.  These Q resins have less organic functionality and are highly hindered to SN2 acid/base chemistry.  This hindrance to nucleophilic attack results in alkaline stability at least. These two methods are the only general approaches I can envision to address the chemical resistance of FAS with silicon-based materials.   

Summary

We have multiple strong solutions to provide hydrophobicity, wetting, and stabilization properties from multiple silicon-based approaches. For oleophobicity and stain resistance we have demonstrated early results with three approaches and are anxious to work on this further. In applications that require chemical stability we have concepts to pursue but no solutions yet.

For the highly critical AFFF formulations, it is likely that low surface tension Silsurf surfactant could be part of future solutions.